1. Field of the Invention
The present invention relates to an infrared/visible integrated imaging apparatus for composing an infrared image with a visible image. In particular, it relates to a video signal mixing device employed in the imaging apparatus for superimposing a visible video signal produced by a visible camera upon an infrared video signal produced by an infrared camera.
2. Description of the Related Art
Infrared radiation is produced by any object having a temperature. Since the infrared radiation is not visible to the human eye, detection thereof requires some physical device such as a thermographic system in which an infrared camera, in conjunction with image processing devices, converts the invisible infrared radiation to a voltage signal which can be viewed directly on a television-like display. The thermographic system has a wide range of applications. For example, in a monitoring system for monitoring extensive areas, such as an electric power generating station or a liquefied natural gas (LNG) storage facility, using infrared cameras, very small changes in temperature in the field of view of the cameras can be detected and displayed on a monitor screen. This monitoring system is applicable to detect a hot spot, namely, a portion with an unusual change of temperature which may indicate an early stage of an accident in the extensive area.
In such a monitor system, there is widely used an infrared/visible integrated camera which combines infrared and visible camera functions. An object to be monitored is viewed along a common optical axis for both infrared and visible observation. This makes it possible to display a composite image composed of visible and infrared images at desired brightness proportions on the same screen. This setup provides more accurate information where the trouble is and how bad it is, than systems separately using a visible camera and an infrared camera. In order to electrically combine an infrared image signal (hereinafter an infrared video signal) with a visible image signal (hereinafter a visible video signal) to form a composite picture, a video signal mixing device is indispensable.
Such an infrared/visible integrated imaging apparatus is disclosed, for example, in Unexamined Japanese Patent Applications No. 52-26268, published on Feb. 26, 1977, proposed by Y. Nakayama, and No. 60-35882, published on Feb. 23, 1985, proposed by Y. Okumura.
FIG. 1 is a block diagram illustrating an example of prior art video signal mixing devices. An infrared camera 1 and a visible camera 2 output an infrared video signal and a visible video signal respectively with respect to the same object (not shown) to be observed. Both cameras 1 and 2 are arranged at positions adjacent to each other to have a common optical axis along which the cameras are focused to the object. The signal levels of the outputs from both cameras are adjusted through an infrared video signal level adjusting circuit 3 and a visible video signal level adjusting circuit 4 with a single control using ganged variable resistors disposed in a level control circuit 6; The adjusted video signals are added to each other through a mixing circuit 5, and converted to a composite signal which is displayed as a composite image of the object on the screen of a TV (television like) monitor 7.
In general, the brightness of an image displayed on the screen of a TV monitor is proportional to the input level of a video signal applied to the TV monitor. A video signal at a high level exceeding a threshold voltage, which is standardized to be 1 volt in Japan, results in an image with an excessive brightness, namely a whitish image. The brightness of the image is saturated with respect to a video signal having a further higher level exceeding the threshold voltage. Hence, the peak level of any video signal should be restricted below the threshold voltage. In contrast, a video signal having a substantially low level, produces a dark image. A specified voltage, below which the resulting image on the screen is invisible, namely a black image, is referred to as a black level. The black level is usually variable and is set to meet the requirement of the user of the apparatus.
In the imaging apparatus, all the infrared images and visible images displayed on the screen of the TV monitor 7, are produced by video signals being scanned line by line like a typical television display. FIG. 2(a) to FIG. 2(c) are prior art diagrams illustrating the waveforms of video signals at different points of the block diagram of FIG. 1. The diagrams illustrate waveforms of infrared and visible video signals for controlling an electron beam of a TV tube (a cathode ray tube) of the TV monitor 7, to repeatedly sweep the beam across the screen of the relevant TV tube. In the diagrams, the video signal level is represented on the (Y-axis) and the signal transmission time for horizontal scanning is taken on the abscissa. One horizontal scanning time interval is represented by H. FIG. 2(a) shows an infrared video signal 30 output from the infrared video signal level adjusting circuit 3. The infrared video signal 30 is composed of a horizontal synchronizing signal 31, an infrared profile signal 32 indicating a high temperature portion of the object, and a black level signal 33 indicating a substantially low temperature portion of the object. FIG. 2(b) shows the waveform of a visible video signal 40 output from the visible video signal level adjusting circuit 4. The visible video signal 40 contains a horizontal synchronizing signal 41, and a visible profile signal 42 indicating an ordinary optical image of the object. FIG. 2(c) shows the waveform of a composite signal 50 output from the mixing circuit 5, being composed of a horizontal synchronizing signal 51, and a composite profile signal 52.
The infrared video signal and the visible video signal are simply superimposed one on the other, making the abovedescribed composite profile signal 52 of FIG. 2(c). The peak level of the composite profile signal 52 is required to be limited below the threshold voltage to achieve a clear and bright image favorable to be observed. As a result, the level of each infrared video signal 30 and that of the visible video signal 40 are further restricted, causing an undesirable image as described later.
The level control circuit 6 including the ganged variable resistors has the advantage that the composite signal 50 composed of an infrared video signal 30 and a visible video signal 40 never exceed the threshold voltage, which is described next.
FIG. 3 is a prior art video signal level chart representing the operation of the level control circuit 6 of FIG. 1. In the figure, the set position of the knob, namely the rotation angle of the knob controlling adjustment the ganged variable resistors, is taken on the abscissa, being indicated by rotation scales from .circle.1 to .circle.6 , and the video signal level applied to the TV monitor 7 volts on the ordinate. It is assumed herein that the level of both of the infrared video signal 30 and the visible video signal 40 are taken maximum, namely equal to the threshold voltage. The dotted line shows the level of the infrared video signal, and the solid line the level of the visible video signal. It is easily understood by observing FIG. 3, that the sum of both output signal levels is always equal to the threshold voltage, and does not exceed the threshold voltage at any rotating scale where the knob of the variable resistor is positioned. This is a favorable feature of the video signal mixing device of the prior art shown in FIG. 2.
In practice, an increase in the brightness of the infrared image is followed by a decrease in that of the visible image and vice versa. This may be advantageous to select a favorable contrast between the infrared image and the visible image in the composite image. However, the ranges of the levels of the infrared video signal and the visible video signal are undesirably limited respectively within a narrow signal level range such as a half of the threshold voltage. The result is that all of the infrared, visible, and composite images are left vague to some degree, thus distinct recognition of images of the unusual points in the field of view, becomes difficult. The cause of the above problem lies in the mechanism that the infrared image is simply mixed with, or added to the visible image.
In view of the above described problem in the prior art there has been a need for an improved infrared/visible integrated device capable of displaying a clearer and more distinct composite image.